Vent assembly

ABSTRACT

A vent assembly, suitable for marine use including, for example, venting an enclosure, such as an engine compartment, on a boat. The vent assembly has substantially the external appearance of a cast or machined, one piece, stainless steel vent, at a cost very little more than a molded plastics vent. A molded plastics vent has a head perforated by a pattern of ventilating slots separated by flanking strips. A decorative, corrosion-resistant, sheet metal cover shell lies tight against the front side of the molded plastics vent and has a pattern of slots and strips mapping substantially on the pattern of slots and strips of the molded plastics vent. Snap-fit fasteners fix the cover shell on the vent so that the cover shell slots are a substantially flush continuation of the vent slots, so as to provide the decorative appearance of a solid metal vent at substantially lower cost and without compromising or through flow capability.

FIELD OF THE INVENTION

This invention relates to slotted vents, and more particularly toslotted vents usable in marine applications such as for ventilating amotor enclosure on a boat.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 5,588,908, assigned to the assignee of the presentinvention, discloses a apparatus for fixing a ventilating hose at anopening in a wall of an enclosure, e.g. for use on a boat in ventinggases from the engine compartment, or venting cooking odor laden airfrom the galley, or admitting air. That apparatus includes a ventfitting comprising a radially extended, slotted head adapted to restagainst the exterior face of the wall to hide an opening therein and atubular body extending from the rear face of the head through the wallopening, to fix the vent fitting and an attached, rearwardly extendinghose to the wall at the opening, to communicate flow through the hoseand slotted head. To accomplish its various purposes, such vent fittingis of relatively complex form. Despite this, the assignee of the presentinvention has succeeded in forming such fittings of molded plasticsmaterial at relatively low cost and has successfully marketed units insubstantial quantities, for example to boat manufacturers.

However, the present assignee has found that for more expensive lines ofboats, boat manufacturers and their buyers prefer more expensive lookingfittings, e.g. stainless steel fittings, rather than less expensivelooking molded plastics fittings.

With that in mind, the present assignee has marketed molded plasticsfittings of this general type with exposed surfaces chromium plated, bya conventional process, to achieve the richer, more expensive appearanceof a polished metal fitting. While generally successful commercially,the present assignee has found that continuing contact, over a longperiod of time, with the elements in a harsh salt water marineenvironment, may attack, and eventually degrade the appearance of, theplated visible surfaces of such fittings.

Accordingly, the present assignee has considered producing such fittingsof corrosion resistant metal, such as stainless steel, as by casting ormachining, and while such product is durable and can maintain a rich,expensive appearance over long periods of use, it is many times moreexpensive to produce than the same product of molded plastics material,and because of this expense would have a very limited market.

The purposes of the present invention include overcoming theseconflicting prior difficulties.

SUMMARY OF THE INVENTION

This invention relates to a vent assembly, suitable for marine usesincluding, for example, venting an enclosure, such as an enginecompartment, on a boat, which vent assembly has substantially theexternal appearance of a cast or machined, one piece stainless steelvent, at a cost very little more than a molded plastics vent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial view of a prior molded plastics vent substantiallyas disclosed in U.S. Pat. No. 5,588,908 assigned to the assignee of thepresent invention.

FIG. 2 is an enlarged cross-sectional view of the FIG. 1 molded plasticsvent.

FIG. 3 is a pictorial view of a cover shell of convexly curved shape,taken from the front side thereof, and in accord with a preferredembodiment of the present invention.

FIG. 4 is a pictorial view of the FIG. 3 cover shell taken from the rearside thereof.

FIG. 5 is a front view of the FIG. 3 cover shell.

FIG. 6 is an enlarged fragmentary cross-sectional view substantiallytaken on the line 6-6 of FIG. 5.

FIG. 7 is an enlarged, fragmentary cross-sectional view substantiallytaken on the line 7-7 of FIG. 5.

FIG. 8 is a pictorial view of a preferred embodiment of a moldedplastics vent for which the FIG. 3 cover shell is intended.

FIG. 9 shows FIG. 6 cover shell installed on a molded plastics vent likethat of FIG. 8.

FIG. 10 shows the FIG. 7 cover shell installed on a molded plastics ventof the type shown in FIG. 8.

FIG. 11 is an enlarged fragmentary cross-sectional view substantiallytaken on the line 11-11 of FIG. 5, and showing the cover shell installedon the FIG. 8 vent.

FIG. 11A is a fragment of FIG. 11, but showing the FIG. 3 cover sheettab at a starting position of installation on the FIG. 8 vent.

FIG. 11B is similar to FIG. 11A, but showing an intermediate position ofinstallation.

DETAILED DESCRIPTION

A vent 40 (FIGS. 1 and 2) is here formed to function as a hose fitting.The vent 40 is formed (preferably molded) of a suitable substantiallyrigid molded plastics material, such as nylon or polypropylene. Themolded plastics vent 40 includes a radially extending head 42, here inthe form of a circular face plate. The head 42 has a radially outwardfacing edge 30 and a continuous peripheral edge portion 43 inboard ofthe edge 30. The head 42 is perforated by a pattern of elongateventilating slots 31 extending axially therethrough and laterally to theperipheral edge portion 43. The slots 31 have end walls 32 at theperipheral edge portion 43. The slots are separated by flanking straps46 defining side walls 33 of the slots 31. The head 42 has front andback sides 20 and 21 respectively separated by the radially outwardfacing edge 30 and to which the slots 31 open.

The molded plastics vent 40 also has a generally tubular base 50extending substantially coaxially from the back side 21 of the head 42at its peripheral edge portion 43. The base 50 is spaced inboard of theradially facing edge 30 by a coaxial, radially extending, rear facing,enclosure wall abutting, annular lip 49 and has a coaxial throughpassage 51 communicating with the slots 31.

In the embodiment shown in FIGS. 1 and 2, the generally tubular base 50is circumferentially segmented to form circumferentially closely spaced,alternating lock tabs 45 and locking fingers 52, all of which extendaxially rearwardly from the head back side at the peripheral edgeportion 43. The lock tabs 45 and fingers 52 are of arcuate,circumferentially extending cross-sectional profile. The rear edges ofthe lock tabs 45 are radially inwardly beveled at 47.

The locking fingers 52 are located slightly radially outwardly from thelock tabs 45. Each locking finger 52 has one or more (here three)axially spaced, radially outwardly protruding, circumferentiallyextending teeth 54 having a rearward and radially outwardly beveled,generally rearwardly facing surfaces 56. The rearwardmost tooth 54 formsthe rear edge of the base 50 and its beveled surface 56 a extends thefull radial thickness of the tooth. The rearward end portion of eachlocking tooth 52 is thickened to provide a slightly radially inwardlyprotruding lip 60. The teeth 54 and lip 60 extend circumferentially thewidth of the locking finger 52. It will be understood that the lockingfingers 52, including their lips 60, are spaced radially outward fromthe outer circumferential plane of the lock tabs 45 and preferably arecircumferentially narrower than the circumferential spaces between thelock tabs 45. Circumferentially spaced, axially extending, reinforcing,rib-like webs 58 protrude radially outwardly from the radially outerface of the locking fingers 52 between the back side 21 of the head 40and the nearest tooth 54.

The molded plastics vent 40 can be used as an end termination for aventilation hose 10 (FIG. 2), a ventilating member to provide a finishedappearance for a hole 14 in a wall 12, or both. In the embodiment shown,the hose 10 is formed of a flexible material in which a coiled wire 64is embedded for reinforcement. The front (leftward in FIG. 2) end of thehose is forwardly inserted into the circumferentially continuous,radially narrow, annular space between the lock tabs 45 and lockingfingers 52 so as to closely approach or abut the back side 21 of thehead 42 of the molded plastics vent 40, as permitted by the limitedradial bending capability at least of the fingers 52. So installed, theradially inwardly protruding lips 60 on the fingers 52 axially fix thehose 10 to the molded plastics vent 40 and prevent unintended axialseparation thereof. With an intermediate portion of the hose 10extending through the hole 14 in the wall 12, rearward movement of themolded plastics vent 40 toward the wall 12, coaxially of the hole 14,inserts the generally tubular base 50 into the hole 14, such that theteeth 54 slide axially rearwardly along the peripheral edge of the hole14, camming and bending the fingers 52 radially inwardly as needed,until the radially extending head 42 of the vent 40 abuts the front faceof the wall 12, thereby completing installation as seen in FIG. 2.

During installation, the tapered rearward surfaces of the teeth 54assist installation of the vent 40 in the wall 12, whereas afterinstallation the forward facing radially planar faces of the teeth 54resist removal of the vent 40 from the wall 12. The radially inwardbending of the locking fingers 52 by the peripheral edge of the hole 14helps the lips 60 of the fingers 60 fixedly grip the hose 10 betweenthemselves and the circumfererentially flanking tabs 45.

To the extent of above discussed, the apparatus is substantially thatdisclosed in the present assignees prior U.S. Pat. No. 5,588,908.

FIG. 8 shows a further prior art vent 140 which may be generally similarto the vent 40 above discussed with respect to FIGS. 1 and 2, except asfollows.

For example, the head 142 (FIG. 8) of the vent 140 has a front side 120of shallow, convexly rounded shape. Thus, the strips 146 havecorrespondingly somewhat convexly rounded front faces 147.

Turning now to aspects of the apparatus more directly dealing with thepresent invention, attention is directed to FIGS. 3-11.

A radially extending cover shell 201 (FIGS. 3-5) is preferably of adecorative, corrosion resistant, relatively stiff material, such assheet metal. In the preferred embodiment shown, the cover shell 201 isof stainless steel sheet. Marine grade stainless steel sheet issufficiently corrosion resistant, even in salt water environments, as tomaintain a decorative appearance over a long period of time.

The cover shell 201 has a continuous peripheral edge portion 205surrounding a central portion 206 comprising a pattern of slots 211separating by elongate strips 206, whose edges define correspondingedges of the flanking slots.

The cover shell 201 has a front face 220 (FIG. 3) and a back face 221(FIG. 4). The cover shell 201 is contoured three-dimensionally tocontinuously snugly cover, in a skin-like manner, the front side of thehead of a corresponding molded plastic vent. Thus, in the embodimentshown, the cover shell 201 of FIGS. 3-5 is three-dimensionally contouredto cover the convex front face 147 of the corresponding vent 140 of FIG.8, it being understood that similar cover shells can bethree-dimensionally configured to similarly fit, in a skin-like manner,the front face of the head of otherwise contoured vents, for examplesuch as the vent 40 in FIGS. 1 and 2 above-described, the front face 20of which is not convexly rounded, but rather is flat (or might beconcavely rounded if desired).

Returning to the particular cover shell 201 shown in FIGS. 3 and 4, samethus is contoured three-dimensionally in a shallow, cup-like shapehaving a convexly rounded front wall 226 incorporating the slots 211 andstrips 216 and a peripheral wall continued therefrom and terminating ina peripheral edge 228.

The cover shell 201 is three-dimensionally contoured so that its backside 221 can continuously abut the front face 147 (FIG. 8) of thesupporting vent 140, in the manner shown in FIGS. 6, 7 and 9-11. Inother words, the cover shell 201 is contoured to fit in a skin-likemanner on the front of the supporting vent head 142.

The pattern of slots 211 and strips 216 of the cover shell 201, as wellas the size, shape and placement thereof, are preferably identical tothose of the slots 131 and strips 146 of the corresponding vent 140.Thus, with the cover shell 201 properly oriented on the front face 147of the vent head 142, the vent head 142 is to be entirely hidden fromthe front by the cover shell 201, as in FIG. 5. Thus, the cover shell201 is to cover the entire exposed portion (the front and periphery) ofthe vent head 142, including the front surfaces of the strips 146, theperipheral edge portion 143, and the radially outwardly facingperipheral edge 130. Thus, at least as a casual observer, the externalappearance of the vent assembly, comprising the molded plastic vent 140and cover shell 201, is to be that of a monolithic metal (e.g.,stainless steel) vent cast from molten stainless steel or machined froma block of stainless steel. This visual effect can be enhanced byreducing the visibility of the molded plastics vent 140. For example, bymolding the plastics vent 140 of a black material with a non-shinysurface, the sidewalls 133 (FIGS. 8 and 9) of the vent head strips 146may visually merge into the shadowed interior reaches of the vent slots131.

In some embodiments, as for example shown in FIG. 8, at least some ofthe vent head strips 146 may be, as seen from the front, several timeslonger than their width. Despite this, such strips 146 may be madeespecially strong by providing same with substantial front-to-reardepth, as seen for example in FIGS. 8 and 9, wherein the strips 146crossing the central portion of the head 142, while laterally longer,are also deeper front-to-rear. On the other hand, the cover shell 201 ispreferably of constant thickness sheet material and indeed very thincompared to any dimension of the vent strips 146.

Thus, to avoid accidental bending after the cover shell 201 is formed,the longer cover shell strips 216 may be reinforced by one or more crossstraps 234 (FIGS. 3-7) spanning one or more of the slots 211 andextending transversely from the flanking strips 216. A given cross strap234 thus acts to prevent twisting (about its length axis) or bending(out of its plane or transversely into or away from a flanking slot) ofthe attached strip 216. So that the cross straps 234 do notsignificantly impede flow through the slots 211, the cross straps 234are preferably minimized in number and length, so that the total frontalarea of the cross straps 234 is small compared to the total frontal areaof the slots 211.

The cover shell 201 can be fixed to the front of the vent head 142 asdesired, for example by adhesive bonding. However, in the preferredembodiment shown, the cover shell 201 is preferably securely butresiliently releasably snap-fitted to the front of the vent head 142 byits own monolithically integrated fastening structure. In the preferredembodiment shown, that fastening structure comprises resilientlybendable, cantilevered tabs 250 (FIGS. 3, 4, 7, 8, 10 and 11) angledsubstantially rearwardly from edges of corresponding slots 211.Advantageously, the tabs 250 extend from ends 251 of one or more of theshell slots 211 and are hence rearwardly bent from inboard edges of theperipheral edge portion 205 of the cover shell 201. Such tabs 250 arethus located to extend into adjacent vent head slots 131 (FIG. 11), hereat opposite ends of the two adjacent longest ones of the vent head slots131. A given said tab 250 (FIG. 11) has a leg 254 extending rearwardlyand positioned to lie along and adjacent the opposed vent head slot end132. Such tab 250 further includes a heel 256 protruding from the freerear end of the leg 254 and positioned to lie laterally beyond the venthead slot end 132 and snugly behind the vent head peripheral edgeportion 143 to lock the cover shell 201 against the front face 147 ofthe vent head 142. Such tab 250 further includes a foot 258 extendingrearward from the radially outboard end of the heel 256 and a toe 260angled rearward and radially inward from the free, rear end of the foot258.

Applicants found that the cover shell 201 can be inexpensively producedfrom stainless steel sheet by a stamping operation which separates thecover shell 201 from the surrounding portion of the parent metal sheet(not shown), removes material to form the slots 211 and flanking strips216, forms the mentioned cup-shaped configuration and forms and bendsout of plane the mentioned tabs.

To install the cover shell 201 on the vent 140, the cover shell is movedrearward toward the front of the vent 140 to cammingly engage the rearface of the toe 260 (FIG. 11A) with the front inboard corner of the ventperipheral edge portion 143 at the appropriate slot end wall 132.Continued rearward displacement of the cover shell 201 causes the ventedge portion 143 to cam the cover shell toe 260 laterally inboard(leftward in FIGS. 11A and 11B), resiliently bending the tab 250leftwardly and allowing the heel 256 to slide rearwardly along thecorresponding vent slot end wall 132 toward and past its FIG. 11Bpartially installed position. Continued rearward displacement of thecover shell 201 brings same into the firm, face-to-face contact with thevent 140, as shown in FIGS. 9-11, whereupon the resiliently bent tab 250snaps its heel 256 (FIG. 11) laterally outward snugly behind the ventedge portion 143. With the tabs 250 (four being provided in theembodiment shown) thus deployed, same firmly and fixedly retain thecover shell 201 tightly against the front of the vent head 142, asgenerally shown in FIGS. 9-11.

Thus installed, it will be noted that the back side 221 (FIGS. 9-11) ofthe cover shell 201 tightly continuously abuts the front side 147 of thevent head 142, with the continuous peripheral edge portion 205 of thecover shell mapping on the head peripheral edge portion 143 and theshell peripheral edge 228 lying flush with the rear facing lip 149 ofthe vent head 142, as seen in FIGS. 9 and 10. Moreover, the pattern ofcover shell slots 211 maps on the pattern of vent head slots 131 inshape, size and relative location (FIGS. 9 and 11) and the cover shellstrips 216 map on the vent head strips 146 in shape, size and relativelocation. Thus, the edges of the shell slots 211 are a substantiallyflush forward continuation of the walls of the corresponding head slots,so as to minimize disruption of air flow through the vent head 142despite the covering thereof by the cover shell 201. In the FIGS. 3-11embodiment, one or more of the cover shell strips 216 is relatively longand narrow, and one or more rigidifying cross straps 234 are integratedin the cover shell 201 and span one or more cover shell slots 211 andcorresponding vent slots 131 but, as indicated in FIG. 9, the crossstraps 234 are minimized in width and number, and so do notsignificantly, if at all, impede air flow through the vent 140.

In a preferred embodiment of the invention, the cover shell 201 wasstamped of grade 316 stainless steel sheet material of thickness about0.5 mm, though use of similar material of thickness in the range 0.2-1.0mm is contemplated.

The use of thicker material may be desirable where the cover shell is ofgreater diameter, has a more flat or otherwise less inherently rigidcross-sectional shape, has longer and/or narrower strips 216, has longerand/or wider slots, or has fewer (or no), narrower and/or longer crossstraps 234. The cover shell thickness may also be varied to compensatefor variations in rigidity as between different cover shell materials.

While the vent 140 and cover shell 201 here shown by way of illustrationhave a pattern of substantially parallel, chordally extending, uniformwidth slots 211, the pattern of slots 211 may be varied as desired inshape, size and relative location within the broader scope of theinvention. For example, the slots 211 could take the form of a pluralityof circular or irregularly shaped holes, and/or nonparallel (e.g.radiating) elongate openings as desired.

In accord with the present invention, the rigidity, frontal impactresistance, and resistance to denting of the vent assembly 140, 201 isprovided primarily by the relatively inexpensive, rigid molded plasticsvent 140 which firmly backs the cover shell 201. Thus, the thickness ofthe cover shell 201, including its strips 211, may be a very smallfraction (e.g. 1/20 to 1/80) of the thickness of the vent head strips.For example, in the embodiment shown, the cover shell strip thickness isabout 1/40^(th) of that of the thickest vent head strips 146.

Thus, the vent assembly 140, 201 according to the present invention, ascompared to a monolithic cast or machined vent of solid stainless steel,is substantially more economical both in terms of substituting lessexpensive plastics material for the great majority of more expensivestainless steel, and in substituting less costly fabrication methods(e.g., relatively inexpensive plastic molding and sheet metal stampingoperations) for more expensive stainless steel casting or solid-blockmachining.

Although a particular preferred embodiment of the invention has beendisclosed in detail for illustrative purposes, it will be recognizedthat variations or modifications of the disclosed apparatus, includingthe rearrangement of parts, lie within the scope of the presentinvention.

1. A vent assembly, suitable for mounting in a hole in a wall of anenclosure, to vent such enclosure, and comprising; a molded plasticsvent having a radially extending head, said head having a radiallyoutward facing edge and having a continuous peripheral edge portioninboard of said edge, said head being perforated by a pattern ofelongate ventilating slots extending axially therethrough and laterallyto said peripheral edge portion, ones of said slots having end walls atsaid peripheral end portion, said slots being separated by flankingstrips defining sidewalls of said slots, said head having front and backsides separated by said radially outward facing edge and through whichsaid slots open, said molded plastics vent having a generally tubularbase extending substantially coaxially from said head back side at saidperipheral edge portion, said base being spaced inboard of said radiallyfacing edge by a coaxial, radially extending, rear facing, enclosurewall abutting, annular lip and having a coaxial throughpassagecommunicating with said slots; a radially extending, decorative,corrosion resistant, sheet metal cover shell lying tight against saidfront side of said vent head, said shell having a continuous peripheraledge portion mapping on said head peripheral edge portion and a patternof slots mapping on said pattern of vent head slots in shape, size andrelative location, said cover shell slots being separated by stripssubstantially mapping on said vent head strips in shape, size andrelative location, an edge of a given cover shell slot being asubstantially flush, forward continuation of the adjacent edge of thecorresponding vent head slot to minimize disruption of airflow throughthe vent assembly, said cover shell including resiliently bendable,cantilevered tabs angled substantially rearwardly from ends of ones ofsaid shell slots and extending into adjacent vent head slot ends, agiven said tab having a leg extending along and adjacent the opposedvent head slot end wall, a heel protruding from the free end of said legbeyond said vent head slot end and snugly behind said vent headperipheral edge portion and locking said cover shell on the front faceof said vent head, a foot extending rearward from the radially outboardend of said heel, and a toe angled rearward and radially inward from thefree end of said foot, said toe having a cover shell installing positionbearing cammingly on the front of said vent head peripheral edge portionto assist snap-fit installation of said tab in its said vent head slot,said cover shell being contoured three dimensionally in a shallow,cup-like shape having a front wall incorporating said slots and stripsand a peripheral wall covering said radially outward facing edge of saidvent head to give the outside appearance of a solid metal vent, saidperipheral wall having an enclosure wall approaching, free, peripheraledge rearwardly approaching the plane of said vent head annular lip. 2.The apparatus of claim 1 in which the axial thickness of said headstrips and cover strips is approximately in the range of ratios 1/20 to1/80.
 3. The apparatus of claim 1 in which said cover is a stainlesssteel stamping.
 4. The apparatus of claim 2 in which said front side ofsaid vent head is convexly rounded and said cover shell iscorrespondingly convexly rounded and lies tight against said front sideof said vent head.
 5. The apparatus of claim 4 in which said head slotsextend substantially parallel and widthwise of said head, said shellslots extending correspondingly parallel and widthwise of said head. 6.The apparatus of claim 1 in which said cover shell has at least onecross strap extending across at least one of said shell slots andbetween flanking strips, said strap connecting and minimizing deflectionof said flanking shell strips.
 7. The apparatus of claim 6 in which thewidth of said strap is at most substantially the width of a saidflanking shell strip to minimize interference with flow through saidhead.
 8. The apparatus of claim 1 in which the thickness of said covershell is in the range 0.2 mm to 1.0 mm.
 9. The apparatus of claim 8 inwhich the thickness of said cover shell is about 0.5 mm.
 10. Theapparatus of claim 1 in which said cover shall is of type 316 stainlesssteel sheet.
 11. A vent assembly, suitable for marine use and the like,for covering a vent opening while permitting air flow therethrough, andcomprising: a vent comprising a radially extending head perforated by apattern of ventilating holes extending axially therethrough, said holesbeing separated by flanking portions defining side walls of said holes,said head having front and back sides through which said holes axiallyextend, said head having a radially outwardly facing edge and aperipheral edge portion inboard of said edge; a radially extending,decorative, corrosion-resistant, cover shell of substantially rigidsheet stock, said cover shell lying tight against the front of said venthead, said shell having a peripheral edge portion mapping said headperipheral edge portion and a pattern of holes mapping on said patternof vent head holes in shape, size and relative location, said covershell holes being separated by flanking portions substantially mappingon said vent head portions in shape, size and relative location, an edgeof a given cover shell hole being a substantially flush, forwardcontinuation of the adjacent edge of the corresponding vent head hole tominimize disruption of air flow through the vent assembly, said covershell being fixed on said vent head, said cover shell forwardly andsidewardly covering said vent head.
 12. The apparatus of claim 11 inwhich said cover shell has a rear facing peripheral edge and said venthead has a rear facing surface substantially flush with said cover shellrear facing edge, the latter two being adapted to substantially abut anenvironmental surface having a vent hole to be covered by the ventassembly.
 13. The apparatus of claim 11 in which said vent is a moldedplastics mass and said cover shell comprises a sheet metal stampingperforated by said holes, said sheet metal stamping including integralrearward extending fastener elements in snap-fit engagement with saidmass.
 14. The apparatus of claim 13 in which said fastener elements aretabs bent rearward from the peripheral portion of the sheet metal atedges of said holes, said tabs being elongate leaf spring-like elements,a given said tab comprising an elongate rearward extending leg receivedat the edge of a corresponding vent head hole, said leg extendingrearward through said corresponding vent head hole and having a heelengaging a rearward surface of said vent head at said corresponding venthead hole end to positively hold said cover shell tight against thefront of said vent head in a positive yet resiliently releasable manner.15. The apparatus of claim 11 in which said vent head has asubstantially dome shaped front portion, said cover shell beingthree-dimensionally shaped to conform to said dome shape, said covershell continuously abutting and covering said vent head forwardly andsidewardly.
 16. The apparatus of claim 11 in which at least ones of saidvent head and corresponding cover shell holes are elongate slots. 17.The apparatus of claim 16 in which said cover shell slots are mutuallyparallel.